Technical Abstract:
Certain exotic plant species are known to engineer soil processes and thereby facilitate their competitive stature and invasiveness. In a well-characterized winterfat (Krascheninnikovia lanata) community in the Honey Lake Valley of northeastern CA, we tested if cheatgrass invasion (Bromus tectorum L.) affects plant nitrogen (N) above-ground tissue concentrations. Three cheatgrass invasion zones were contrasted; invaded for 1, 4, and >10 years. Within each invasion zone, 4 replicate areas were identified and from each area four characteristic species were collected and total aboveground tissue analyzed for total carbon (C), total N, and '15N. Tested species included cheatgrass, winterfat, freckled milkvetch (Astragalus lentiginosus), and Indian ricegrass (Achnatherum hymenoides). As time since invasion by cheatgrass increased, tissue N increased and C/N decreased significantly for cheatgrass and winterfat. Time since invasion had no effect on tissue N or C/N for Indian ricegrass or milkvetch. Except for cheatgrass, time since invasion significantly affected '15N, declining significantly for winterfat and increasing significantly for Indian ricegrass and milkvetch. Soil nutrient data collected at the site over 7 years, suggest that cheatgrass invasion fosters an increase in the N-cleaving enzyme amidase, which may be, in part, responsible for elevated N availability. Moreover, the greatest increase in N availability, fostered by cheatgrass invasion, occurs in lower soil horizons. These findings combined with '15N data suggest cheatgrass is accessing perhaps new pools of N thereby increasing N availability to itself and some plants able to survive the invasion. Loss of N mineralization potential upon long-term invasion suggests access to new N pools may be exhausted.